This invention relates to a circuit substrate, more particularly, to a hybrid-type circuit substrate comprising an alumina plate and, bonded thereto an aluminum nitride plate which exhibits an excellent heat dissipating characteristic, being free from generation of crack and the like at the bonded portions of these plates.
As a circuit substrate, there has generally been employed an inexpensive alumina (A1.sub.2 O.sub.3) substrate, on which a circuit is formed and various elements including a semiconductor device is mounted.
However, in recent years, with the larger output of a semiconductor device, generation of heat from the device has become larger and the above-mentioned Al.sub.2 O.sub.3 substrate causes a problem in that it is not necessarily satisfactory with respect to the heat dissipation.
In order to solve the problem, an aluminum nitride (AlN) substrate having an excellent heat dissipating property may be employed in place of the A1.sub.2 O.sub.3 substrate for constituting a circuit substrate. However, since the former is much more expensive than the latter, it can not be said that the former is suitable in practical use.
Then, it has recently been tried to improve the heat dissipation from an element and reduce the cost thereof by preparing a circuit substrate with an Al.sub.2 O.sub.3 plate in combination with an AlN plate and placing, only on the AlN plate, an element which exhibits particularly larger heat generation (see Japanese Provisional Patent Publication (KOKAI) No. 7647/1986).
As the construction of such hybrid-type circuit substrate, there may be mentioned a construction as shown in FIG. 1 in which an aperture portion 1a is arranged at a desired portion of an Al.sub.2 O.sub.3 plate 1, an AlN plate 2 having a shape corresponding to the aperture portion 1a is fixed therein, and the side faces of the AlN plate 2 is bonded to the inner wall faces of the aperture portion 1a; a construction as shown in FIG. 2 in which, onto a peripheral portion around an aperture portion 1a on an under surface of an Al.sub.2 O.sub.3 plate 1, a peripheral portion of a surface of an AlN plate 2 is bonded; etc.
Also, when plates of ceramics including Al.sub.2 O.sub.3 and AlN are to be bonded to each other, metallization and soldering are usually conducted preliminarily before bonding. However, as disclosed in KOKAI No. 7647/1986 and EP-0142673 Al, a ceramic plate is bonded via a buffering layer to another ceramic plate or a metal plate without any metallized and soldered layers.
However, it was found by the present inventors that, since a stress is caused at the time of use due to the difference between the thermal expansion coefficient of Al.sub.2 O.sub.3 and that of AlN, cracks are sometimes caused at the bonded portion and, in particular case, the AlN plate falls off and when the circuit substrate has been hermetically sealed to form a package, the reduction of airtightness thereof is inconveniently caused.
As mentioned above, it was experimentally proved that the Al.sub.2 O.sub.3 /AlN hybrid-type circuit substrate tends to generate crack in use due to the difference of the thermal expansion coefficients of both plates.